1. Field of the Invention
This invention relates to lead-in wires for coupling audio modulated, broadcast band signals carried on a transmission line to car radios in a drive-in movie theater system and more particularly to such a lead-in which prevents audio disturbances resulting from interference by broadcast band signals picked up by the transmission line.
2. Prior Art
Most present drive-in movie theaters employ sound systems in which an audio signal from the film sound track is carried to in-car speakers or earphones through wire.
It has previously been proposed to transmit the audio signal to special receivers located in the car but the high cost of these receivers renders this solution uneconomical. It has also been proposed to broadcast a special signal which would be picked up by the patron's car radios but legal restrictions on such broadcasts have rendered this approach impractical.
Systems have also been proposed wherein the patron's car radio receivers are directly wired to a broadcast band transmitter modulated by the audio signal from the sound track. Connection is achieved by attaching a lead-in wire, coupled at one end to a transmission line carrying the audio modulated signal, directly to the car antenna with an alligator clip or the like. This system is highly susceptible to interference caused by strong broadcast band radio signals picked up by the transmission line, or the lead-in wire, which effectively act as extensions of the receiver antenna. These extraneous signals interfere with the audio modulated sound track signals to produce a beating sound in the audio output of the receiver.